Cellulosic structure and process for producing same



Patented 4, 1941 PATENT OFFICE CELLULOSIC STRUCTURE AND PROCESS FOR PRODUCING SAME i John C. 'Woodhouse, Wilmington, Del., asslgnor to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware 10 Claims.

This invention relates to celluloslc structures, especially those of pellicular nature. More particularly, it relates to a method for producing flexible and durable cellulosic pellicles by treating the same with softening agents, and the products resulting from such treatment.

In the manufacture of cellulosic pellicles of the type precipitated from aqueous alkaline cellulosic solutions, such as regenerated cellulose pellicles, including sheets or films, caps, bands, continuous tubing, artificial straw and the like, it has long been customary to incorporate into the cellulosic pellicle a softening agent in order to maintain it in a flexible condition. Such cellulosic pellicles, when free from a softening agent, are known to be quite brittle, and while a softenerfree pellicle may find certain uses, a softened and therefore flexible sheet or film is much more gen-' erally useful. Heretofore, as softeners for regenerated cellulose, and similar water-sensitive cellulosic pellicles, the art has applied such substances as ethylene glycol, propylene glycol, diethylene glycol, formam ide, glycerol, and the like, but of these, by far the most important is lyc- .erol which has hitherto known no equal as a softening agent of general utility forproducing flexible, transparent, and durable water-sensitive cellulosic pellicles such as those of regenerated cellulose. Many attempts have been made 80 to find a softener for water-sensitive cellulosic pellicles which will be as cheap as and which will have the generally useful softening characteristics of glycerol.

It is, therefore, an object of this-invention to provide a new and useful method for producing softened water-sensitive cellulosic materials possessing desirable physical properties. It is a fur. ther object of this invention to provide watersensitive cellulosic materials having a sottening agent associated therewith, which softening agent. will not interfere with any subsequent treatment of the cellulosic pellicles, such as the application of surface coatings, ink, coloring matter, adhesives, or the like. It is a still further 45 object of this invention to provide a rigid, flexible, transparent, durable cellulosic pellicle, such as a sheet, film, or tube of regenerated cellulose in which the softening agent consists in whole or in part of a water-soluble amide of a hydroxy acid containing 3 or 4 carbon atoms. Other objects of the invention will appear hereinafter.

By the term "rigidity or its equivalent as used herein is meant that property which is the opposite of limpness; in other words, rigidity in 88 the sense that .the pellicle can be advanced in a wrapping machine or similar apparatus without curling, crumpling or otherwise fouling the machine.

The term durability" is used to define resistance to shock or rough handling. Thus, for example, bags may be fabricated from the pellicle and filled with some standard material such as a certain weight of dried beans, and the package sealed, whereupon the durability may be measured against a known standard by dropping the package under standard conditions and noting the resistance of the wrapper to breakage.

The objects of this invention can be attained, in general, by treating a regenerated cellulose pellicle, or a like water-sensitive cellulosic pellicle, with a softening agent comprising a water-soluble hydroxy acid amide of the following structure:

in which R designates an alkyl radical of 2 or 3 carbon atoms, and R1 and R2 designate a hydrogen or a non-hydroxylated hydrocarbon radical. As specific examples of such compounds may be mentioned lactamide and alpha-hydroxy isobutyramide, betahydroxy iso-butyramide, alpha-hydroxy n-butyramide, betahydroxy n-butyramide, gamma-hydroxy n-bu-tyramide, and betahydroxy propionamide. The excess softening agent is then removed from the pellicle, and the pellicle so treated is dried.

In the manufacture of regenerated cellulose sheets or films, as for example by the viscose process, the cellulosic dispersion is formed into a sheet or film by passing the same through suitable apparatus into coagulating and/or regenerating baths from which it is finally obtained in the form of a. continuous cellulosic sheet which is customarily led in a continuous manner through a series of purifying, bleaching and washing operations. Just prior to the drying operation which is also usually continuous, coordinating with the sheet forming and purification'operations, the film is passed through a bath containing a softening agent in such concentration that after the excess liquid has been removed from the surface of the sheet by suitable squeeze rolls and the excess moisture removed by passage through the dryer, the final sheet can be wound upinacontinuous fashion and will contain an appropriate predetermined amount of softening agent.

The commercially available dry regenerated cellulose film contains approximately 4% to 8% moisture. Depending upon the particular uses for which the product is destined, the softening agent in the sheet or film, usually glycerol, may vary from about 8% to 25%, based on the combined weight of the cellulose and softening agent in the sheet. 7 I

The total amount of softener in the film is controlled primarily by the total amount of softening agent in the treating bath; film thickness, rate of passage through the bath, temperature of the bath, etc., may also contribute thereto to a certain extent. The regenerated cellulose pellicle, when reaching the treating bath, is in a highly swollen and hydrated condition and usually the cellulose of the ellicle is associated with 300% or more of water. This highly swollen and wet pellicle, usually referred to as the gel sheet, is impregnated with the treating bath. Because of the large amount of water associated with the cellulose, it is apparent that the removal of this water during the drying operation will concentrate the softening agent with respect to the cellulose content of the pellicle.

Thus it is that if a regenerated cellulose pellicle containing about total softener is desired as a final product, the concentration of softener in the treating bath will be adjusted to approximately one-third that value, or about 5%. This is true when the softening agents are, like glycerol and the improved agents of this invention, substantially non-volatile and are not vaporized during the drying operation. It is obvious that if a mixed softener is used, the components of which are appreciably volatile during the drying operation, then the softener concentration of the treating bath must be increased sufi'iciently to compensate for the loss thereof during the drying step.

It has now been discovered that amides of the type indicated above are extremely well suited as softening agents for water-sensitive cellulosic pellicles, that is, pellicles produced from aqueous alkaline cellulosic solutions or dispersions coagulated in an acid coagulating bath, especially those of regenerated cellulose, and that they may be used as substitutes for glycerol in this capacity. Furthermore, pellicles of regenerated cellulose softened with the materials of this invention possess desirable properties which are not found in similar pellicles in which the sole fiexibilizing and softening agent is glycerol.

The following examples will illustrate the practice of the invention, but it is to be understood that they are in no way limitative of the principles of the invention:

Example I A sheet or film of gel regenerated cellulose in which. the final dry thickness will be about .0009" is immersed in or drawn through an aqueous bath containing lactamide, the concentrations of the softening agent being adjusted so that the final dry product will contain about 6.3% moisture and 14% lactamide, based .on the combined weight of cellulose and softener in the film. The sheet or film is allowed to remain in contact with the bath until thoroughly impregnated, whereupon it is removed from the bath, the excess liquid drained off or removed by suitable squeeze rolls, blotters, or the like, when the film can be dried in the usual manner down to the final moisture content. indicated above.

" When tested at a relative humidity of about 35% (24 'C.) or even at relative humi itles as low as 15-22% at ordinary temperatures, the durability of the film is found to be at least equal to that of a similar regenerated cellulose film softened with about 14% glycerol.

Example II A sheet or film of gel regenerated cellulose in which the final dry thickness will be about .0009" is treated in the manner described in Example I, using, however, a treating bath having a composition capable of providing a final film containing about 6.3% moisture and 4% alphahydroxy isobutyramide. As before, the properties of the resulting dry film will be equal to or superior to those of a similar film containing 14% glycerol as the softening agent.

Example IV s A sheet or continuous film of gel regenerated cellulose similar to that of Example I is treated in the manner described in Example I using, however, a treating bath of the following approximate composition:

Per cent by weight Glycerol 3 0 Alpha-hydroxy isobutyramide 3.0 Water 94.0

The ratio of alpha-hydrpxy isobutyramide to glycerol in this example is approximately 1:1. The film obtained will contain total softener in the amount of about 18%, based on the combined weight of the cellulose and softener, and it will be equal or superior in its physical properties to a similar regenerated cellulose film which would be obtained by treatment with a bath containing 3.0% glycerol alone, or even a bath containing 3.0% alpha-hydroxy isobutyramide alone.

Example V A sheet or continuous film of gel regenerated cellulose similar to that in Example I is treated in the manner described in Example I using, however, a treating bath of the following approximate composition:

fPer cent by weight Lactamide 4.0 Alpha-hydroxy isobutyramide 2.0 Water 94.0

The physical properties of the treated and dried film are superior to thdse of the similar film containing 14% glycerol as the softening agent.

The following example illustrates a method which may be employed when a relatively volatile cellulose softener is admixed with an hydroxy acid amide of the present invention, and compensation is made for loss during the drying operation:

Example VI A sheet or continuous film of gel regenerated cellulose such that the final dry thickness will be approximately .0009" is immersed in or passed through a treating bath of the following approximate composition:

In this bath the ratio of alpha-hydroxy isobutyramide to ethylene glycol is 1 to 1. The film is treated as described in Example I and yields a final product containing about 6.3% of moisture, 7.5% of alpha-hydroxy isobutyramide and 6.0% of ethylene glycol. The ratio of acid.

amide to ethylene glycol in the dry film is thus about 1.25 to 1.0, the amount of ethylene glycol having been decreased by loss during the drying operation. The dried film is subsequently provided with a surface coating which may be of any type desired. Thus, for example, it may be provided with a moistureproofing coating which may conveniently contain a cellulose derivative, a plasticizer. a resin or blending agent, and a moistureproofing agent such as a wax or waxlike material. The surface coating in this case has the double purpose of providing a moistureproof product and of preventing loss by evaporation of the relatively volatile ethylene glycol. Even when tested under such drastic conditions as described in Example I, the product is transparent, flexible and more durable than a similar film softened with 14% glycerol alone or 14% of alpha-hydroxy isobutyramide alone.

Films obtained in the preceding examples are found to possess, in addition to good transparency, rigidity, flexibility, durability, and excellent surface characteristics.

It is obvious that in the above examples the concentration of total softening agent in the treating bath may be suitably varied according to the base being treated in order that the final product will have the appropriate amount of softener.

As a base, the invention contemplates the use of any water-sensitive cellulosic structure, par. ticularly of pellicular nature such as a sheet or film, artificial straw, caps, bands, or continuous tubes, such as may be obtained by the coagulation and/or regeneration from an aqueous cellulosic dispersion in accordance with the procedure customary to the art. Thus, the invention comprehends the use of regenerated cellulose pellicles such as may be obtained from the viscose or cuprammonium processes, as well as pellicles composed of glycol cellulose, cellulose glycollic acid, lowly esterified or lowy etherified cellulose derivatives where there is only one ether or ester group associated with several cellulose units, and other cellulosic structures of similar characters.

Similarly, various types of paper, especially of the glassine type, may be used as base mateterials. As a matter of convenience, the invention has been described in terms of its application to the softening of regenerated cellulose pellicles such as those sheets or films of regenerated cellulose which are suitable for use as wrapping tissue, it being understood that the scope of the invention includes any of the other bases set forth above and that the invention is equally applicable thereto.

All of the examples given have been set forth in terms of a sheet or film of gel regenerated cellulose (gel regenerated cellulou being defined as a water-swollen regenerated cellulose which' has never been dried; hence. that product which is obtained from the wet end of the casting machine). Obviously, this is the more practical way of practicing the invention, since the soften- 5 ing agent is customarily incorporated into the sheet or film while the latter is in the gel state and during the normal course of manufacture. Obviously, if one desires to impregnate an already dried regenerated cellulose film with a softener of the character described, it is possible to rewet the dried film so as to render it highly swollen, whereupon it may be treated with treating baths similar to those described. but of composition suitable for obtaining the final product desired. In the same way. any of the sheets set forth as equivalent to regenerated cellulose sheets can be substituted in the specific examples.

The invention has been described in part in terms of the use of glycerol in combination with 20 the softeners of the present invention. It is to be understood that other similar known cellulose softeners may be substituted for the glycerol and among those may be mentioned ethylene glycol, diethylene glycol, triethylene glycol, formamide, carbamide,- and other substances known to the art as softeners for water-sensitive cellulose structures.

It is also to be understood that various mixtures of the newly described softening agents may be employed with single softeners of the prior art or with mixtures of the latter, depending upon the type of final film desiredand the purposes for which the film is to be used.

If a colored cellulosic pellicle is desired, it may 85 be obtained in any of the ways commonly known in the art, including the passage of the sheet or film through a bath containing a suitable dyestuff. If desired, the dyestuif may be added to the bath used for introducing the softening agent. 40 In the same way, after the film has been treated with the softening agent, it may be subjected to any of the customary after-treatments such as sizing or coating, or the like, which may be customarily given to cellulosic pellicles of the 45 type described, in just the same manner that a glycerol softened regenerated cellulose film, for example, may be treated.

The instant invention offers numerous advantages over the prior art means of softening cel- 5o lulosic pellicles of the type described. Since the softening agents of the present invention are substantially solid substances at ordinary temperatures, the control of concentrations used for treating baths is greatly simplified. Because 65 of their crystalline nature, the softening agents are available in highly purified condition and consequently improve the quality ofthe final product. However, unlike many crystalline compounds, they do not crystallize after incorpora- 60 tion in the sheet in the concentrations useful for softening. They can be obtained synthetically at a very low cost in comparison to the cost of known softeners and they are available in un-' limited quantity independent of a by-product 65 source.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood 70 that the invention is not to be limited thereto except as set forth in the following claims.

I claim:

1. A flexible. durable water-sensitive material of the type precipitated from alkali metal-hy- 1| droxide solutions from the group consisting of regenerated cellulose, glycol cellulose and cellu- .2. A flexible, durable regenerated cellulose.

structure, said structure containing, as a softener therefor, a water-soluble hydroxy acid amide having the structure in which R designates an alkyl radical containing not less than 2 and not more than 3 carbon atoms, and R1 and R2 designate a member of the group consisting of hydrogen and non-hydroxylated hydrocarbon radicals.

3. A flexible, durable water-sensistive material of the type precipitated from alkali metal hydroxide solutions from the group consisting of regenerated cellulose, glycol cellulose and cellulose glycolic acid, said material containing, as 5 a softener therefor, lactamide.

4. A flexible, durable water-sensitive material of the type precipitated from alkali metal hydroxide solutions from the group consisting of regenerated cellulose, glycol cellulose and celluloseglycoli'c acid, said material containing, as a softener therefor, alpha-hydroxy isobutyramide.

5. Regenerated cellulose softened with lactamide. v

6. Regenerated cellulose softened with alphal5 hydroxy isobutyramide.'

7. Regenerated cellulose softened with 8% to 25% of a hydroxy butyramide.

8. Regenerated cellulose softened with 8% to 25% of a hydroxy propionamid'e.

9. The product of claim 1 when the softener is a hydroxy butyramide. 10. The product of claim 1 when the softener is a hydroxy propionamide.

25 JOHN WOODHOUSE. 

